The present invention relates generally to a ceramic scintillator and a method for producing the same. More specifically, the invention relates to a ceramic scintillator suitable for a radiation detector for detecting radioactive rays, such as X-rays, xcex3-rays and neutrons, and a method for producing the same.
A scintillator (a fluorescent material) is a material for radiating electromagnetic waves, which have wavelengths in the visible or nearly visible spectral regions, by the stimulation of radioactive rays, such as X-rays. The scintillator is used for, e.g., a radiation detector of an X-ray CT (Computed Tomography) equipment as a scintillation counter. FIG. 1 shows a schematic construction of a radiation detector of an X-ray CT equipment.
In this figure, the radiation detector comprises a scintillator 1, a visible light reflective film 3 for covering the whole surface of the scintillator 1 except for a part thereof, a photodiode 2 placed on the part of the scintillator which is not covered with the reflective film 3 and optically coupled with the scintillator 1 through an adhesive layer 4, and an output terminal 33. The reflective film allows the transmission of radioactive rays and reflects visible light. Output electric current from the photodiode is produced in accordance with the quantity of visible light emitted from the scintillator by the stimulation of X-rays being incident thereon through an object in a direction of the arrow in the figure. The output current is transferred to a computer for data accumulation and processing.
For the scintillator 1 of this type thus provided in the radiation detector, it is required that the conversion efficiency from X-rays to visible light should be high, that the afterglow of the scintillator should be short, that the light output of the scintillator should not be reduced (should be stable) under long-term or repeated exposure of X-rays.
Conventionally, for such a scintillator, there are known single crystalline substances, such as cadmium tungstate (CdWO4), sodium iodide (NaI) and cesium iodide (CsI); rare earth oxide ceramics; rare earth oxysulfide ceramics.
Representative rare earth oxysulfides are praseodymium-, terbium- and europium-activated rare earth oxysulfide (M2O2S:Pr, M2O2S:Tb and M2O2S:Eu; M denotes at least one of elements of La, Gd, Y and Lu ). In particular, praseodymium-activated gadolinium oxysulfide ceramics, such as Gd2O2S:Pr and Gd2O2S:Pr,Ce, have a large X-ray absorption coefficient and a short afterglow time of light emission, so that these ceramics are preferably used for an X-ray detecting scintillator.
This type of ceramic scintillator is desirable to be transparent or translucent in order to obtain a high detection sensitivity. To meet this request, the above described rare earth oxysulfide ceramics are prepared by sintering raw material powder under high pressure at high temperature, using, e.g., the hot pressing or the hot isostatic pressing (HIP) techniques. A ceramic scintillator is then obtained by cutting the resulting sintered ceramic ingot so as to have desired shape and size with the blade-saw or the wire-saw.
As-sintered scintillators thus prepared are usually body-colored in gray to blackish hue, which may be related to microscopic distortion or slight deviation in chemical composition from the stoichiometric one induced inside the ingot during the sintering. The coloring reduces the light output efficiency, since a part of emitted light inside the ceramics is absorbed by the colored body before going out of it. Furthermore, the distortion and deviation in the composition make afterglow longer and reduce the light output efficiency of the scintillator under long-term or repeated X-ray irradiation. In order to remove such coloring in the scintillator, it was attempted to carry out a heat treatment at a temperature of 800 to 1400xc2x0 C. in an atmosphere of a mixed gas of hydrogen or hydrogen sulfide and an inert gas. However, it was not possible to completely remove the coloring.
Moreover, in the process cutting the scintillators out of the ingot, a scintillator surface layer of 3 to 5 xcexcm is crushed and damaged. This surface layer is also colored in brown-orange hue, the hue intensity depending on the roughness of the sawing procedure. The hue is almost the same as that which is produced in the raw material powder under strong milling and characteristic of gadolinium oxysulfide. This coloring also reduces the optical output efficiency of the scintillator. In order to remove the coloring of the surface layer, it has been proposed for the surface layer to be washed out with an etching reagent. It was possible to remove the coloring of the scintillator surface by this method, but it was not possible to remove the coloring induced inside the scintillator during the sintering. Moreover, the etching reagent sometimes penetrates deep into grain boundaries to form grooves of about 30 to 50 xcexcm on the surface of the scintillator. These grooves confine the emitted light within them, causing problems of deterioration in the optical coupling and the optical output when a photo-detector, such as a photodiode, is incorporated.
In order to remove both of the internal and surface colorings caused during sintering and by the cutting process, a heat treatment was proposed to carry out in an inert atmosphere containing oxygen of 0.5 to 200 ppm (see Japanese Patent Laid-Open No. 6-201834).
According to this method, it is possible to improve light output efficiency, probably due to removal of the microscopic distortion in the ceramics and to the reduction in the number of oxygen deficiencies in the scintillator crystalline lattices by supplying oxygen. However, it is not possible to remove completely the long part of the afterglow and to improve the stability under X-ray irradiation, probably since it could not reduce the number of sulfur deficiencies in the scintillator. In addition, the surface is covered with an M2O3 layer or M2O2SO4 layer which have a long afterglow and low luminous efficiency. M is yttrium (Y), Lanthanum (La), gadolinum (GD) or the like.
The addition of these inferior properties of the surface layer cancelled partly the improvement in the properties performed inside the scintillator by this method. Furthermore, variation in the performance among scintillator pieces remains large, probably since the sulfur deficiency remains unevenly inside the ceramics ingot.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a ceramic scintillator, which has a high optical output characteristic, a short afterglow and an excellent stability, by removing the internal coloring produced during the sintering of a ceramic and removing the surface coloring produced during cutting.
In order to accomplish the aforementioned and other objects, according to a first aspect of the present invention, there is provided a method for producing a ceramic scintillator, the method comprising the steps of: producing at least one of SO2 and SO3; and allowing the at least one of SO2 and SO3 to react with the sintered body of the rare earth oxysulfide at a temperature of 900 to 1200xc2x0 C. to form a rare earth oxide phase on the surface of the sintered body.
According to a second aspect of the present invention, the ceramic scintillator producing method further comprises the steps of: placing a sintered body of a rare earth oxysulfide and a sulfide material in a substantially sealed container containing oxygen; and heat-treating the sintered body and the sulfide material at a temperature of 900 to 1200xc2x0 C.
By adopting the ceramic scintillator producing method according to any one of the first and second aspects, it is possible to remove the oxygen deficiency and sulfur deficiency produced during sintering, so that it is possible to improve the optical output of the scintillator, to reduce the afterglow thereof and the deterioration thereof under repeated X-ray irradiation.
According to a third aspect of the present invention, a ceramic scintillator comprises a sintered body of a rare earth oxysulfide, and a rare earth oxide phase formed on the surface of the sintered body, wherein a rare earth oxysulfide phase or the rare earth oxide phase is dispersed on the surface of the sintered body. With this construction, it is possible to improve the luminous efficiency of the scintillator and to reduce the afterglow thereof and the deterioration thereof under X-ray irradiation.
According to a fourth aspect of the present invention, a ceramic scintillator comprises a sintered body of a rare earth oxysulfide, and a rare earth oxide phase formed on the surface of the sintered body, wherein the mean surface roughness Ra is greater than or equal to 0.01 xcexcm and smaller than or equal to 0.8 xcexcm. With this construction, it is possible to improve the matching characteristic of the ceramic scintillator with a photodiode or the like, which is optically coupled with the scintillator, so that it is possible to improve the optical output characteristic of the scintillator.